DNA fingerprint techniques and STR marker typing were applied to two populations of rhesus macaques. One troop [Group R] was resident on a 15 ha island, Cayo Santiago [CS], off the coast of Puerto Rico, while the other troop studied [Group M] inhabited a one-acre outdoor corral at the Sabana Seca Field Station [SSFS] in Puerto Rico. The youngest males identified as sires in Group R were six years old, while the youngest males identified as sires in Group M were four years old. In the adolescent male cohort followed from four to six years of age in Group R, the highest ranking individual was the only one who sired offspring by the age of six years. When he sired offspring at 5.5 years of age, he was the largest of his peers, had the highest testosterone concentrations, and possessed the most voluminous testicles. Prime age males, i.e., 9-12 years old, sire significantly more offspring than expected based upon their age distribution within Group R, while peak progeny production coincided with the ages of four to six among the males in Group M. Between 1991 and 1995, the variance in annual progeny production among sexually mature males in Group M ranged from none to eleven. The coefficient of variation in annual offspring production averaged 193% or almost three times that of females [CV-68%] over the same five-year period. OBJECTIVE: To document the proximate mechanisms responsible for variation in reproductive success in male rhesus macaques by determining the relationships among hormones, behavior, morphology, and paternity. FUTURE DIRECTIONS We plan to initiate noninvasive fecal endocrine analysis on the target populations in order to more precisely establish links among physiology, behavior, and paternity. Oru information will be used to establish the genetic history for the endocrine monitoring in the next phase of this project. Samples collected on wild monkeys will be analyzed at the WRPRC, using that center's resources. KEY WORDS growth, development, hormones, puberty, dominance rank, paternity